[wizard]

Ger,
How about length of axis in the DIY machines. Do you agree that 4 feet is pretty much the limit for a screw drive? Given a 72 inch screw minus the allowance for the gantry width. But with a belt drive, longer is possible?

4 feet is really nothing for a leadscrew. You do need to consider larger diameter leadscrews for long stretches. In that regard the best thing you can do for a design is to go to the experts, that is the lead screw manufactures and refer to their engineering data which many freely distribute. Within that information you will likely find tables for span capabilities, max speeds for both supported and unsupported ends, load capabilities and the like. As the span increases your have to increase the screw diameter which adds to the expense quickly. As things get larger you need to watch out for the rapid increase in expense of other parts of the machine. Bearings, mounting blocks, couplings, servos and the like rapidly increase in cost.

The attached image is my inspiration for joining this thread. The owner posted this machine in the Vectric forum. I hope it's ok with him that I repost it here.

I can't image buying an acme screw long enough to span that distance pictured.
Dave

The span isn't a problem. As noted you would need to make sure the proper lead screw is selected. If your goal is a machine to handle wood products I'd seriously consider a belt drive.